Laryngeal Mask Airway (LMA) devices are illustratively described in U.S. Pat. Nos. 4,509,514, 4,995,388, 5,249,571, 5,282,464 and others. As airway devices per se, they have been successful in use, differing from the classical endotracheal tube (ETT) in that an LMA features an inflatable ring or cuff which forms a seal with a patient's airway by surrounding the opening into the glottis, instead of passing through the glottis and vocal cords into the windpipe (trachea).
One consequence of these arrangements is that the vocal cords remain free to close when using an LMA, whereas they cannot do so if the patient has been intubated with an ETT. In practice, this means that if anaesthesia is or becomes insufficient and the patient begins to wake up during surgery, pain can stimulate the vocal cords to close, leading in turn to an inability to ventilate the lungs. This usually triggers an alarm on the ventilator (the machine for driving respiratory gases into the patient's lungs), thus alerting the anaesthetist that something is wrong. Such a sequence can occur when using an LMA but it cannot occur when using an ETT; as a result, in use of an ETT, insufficient anaesthesia may progress to the point that the patient becomes aware during anaesthesia but is unable to communicate his awareness to the anaesthetist.
Use of an LMA thus has an advantage over use of an ETT in acting indirectly as a monitor of anaesthetic depth. However, closure of the vocal cords is undesirable because it may result in insufficient oxygen delivery, and it would be preferable to detect the onset of patient pain and/or awareness at an earlier stage, so that corrective action can be taken before oxygen delivery is jeopardized. To some extent, in the inventor's experience, earlier detection is possible, using one or more of the following methods:
1. Setting the pressure alarm on the ventilator at a level only slightly above that recorded when the patient is fully anaesthetized; PA1 2. Measuring the inflation pressure in the LMA cuff, because LMA-cuff pressure is related to the tonic contractile state of muscles surrounding the cuff; PA1 3. Noting any change in the shape of the patient's expired carbon-dioxide tracing, since it is normal practice to continuously measure expired carbon dioxide during anaesthesia; and PA1 4. When suitable equipment is available, measuring the inspiratory and expiratory flow-volume loops, and noting any alteration in loop patterns.
All of these methods are subject to false positive interpretation, and a more specific test of laryngeal muscle activity would be useful.